This invention is generally directed to improvements in television receivers, and particularly to an improved tuning system for television receivers.
Television receivers of the type under consideration frequently include a frequency synthesizer for tuning the receiver's local oscillator to the nominal carrier frequency of a selected television channel. Generally, the frequency synthesizer compares the local oscillator frequency to a reference frequency associated with the selected channel, and then varies the local oscillator frequency until it equals the reference frequency. Presumably, the receiver will now be correctly tuned to the selected channel.
This method of tuning the receiver relies, of course, on the frequency of the incoming television signal being equal to its nominal or standard frequency. However, when the incoming signal is derived from a video game or other non-standard signal source, the incoming frequency may be non-standard. In that case, a receiver having a frequency synthesizer but no AFC or fine tuning knob cannot tune the receiver to the frequency of the non-standard incoming signal.
Some television receivers do include an AFC control but lack a fine tuning knob which the customer can adjust. However, the AFC control cannot generally be designed to cope with all off frequency signals because of the characteristics of television broadcasting systems. Therefore, even with an AFC control, the receiver may not be able to properly tune to all non-standard signals, such as those which are off frequency by 1.25 megahertz or more.
In addition, when poor signal conditions are present, it is sometimes desirable to tune the receiver off frequency to obtain a crisper picture. Of course, this cannot be done without a manual fine tuning knob.
The inclusion of both AFC and a manual fine tuning knob overcomes the problems described above, but only at a cost penalty since both are relatively expensive, particularly for low cost television receivers. The present invention overcomes these problems in a relatively inexpensive manner.
Another problem associated with conventional frequency synthesizers is that their digital circuits tend to inject noise into the receiver's television image. Costly filters have generally been required to reduce such noise to acceptable levels. The present invention also reduces this noise problem without the need for expensive or elaborate filters.